The present disclosure generally relates to semiconductor ion implantation, and more specifically, to methods for selectively implanting silaborane molecules into semiconductor work pieces.
Ion implanters can be used to treat silicon wafers by bombardment of the wafers with an ion beam. One use of such beam treatment is to selectively implant the wafers with impurities and/or dopants of a controlled concentration for fabrication of integrated circuits.
A typical ion implanter includes an ion source, an ion extraction device, a mass analysis device, a beam transport device and a wafer processing device. The ion source generates ions of desired atomic or molecular species. These ions are extracted from the source by an extraction system, typically a set of electrodes that energize and direct the flow of ions from the source. The desired ions are separated from byproducts of the ion source in a mass analysis device, typically a magnetic dipole performing mass dispersion of the extracted ion beam. The beam transport device, typically a vacuum system containing an optical train of focusing devices transports the ion beam to the wafer processing device while maintaining desired optical properties of the ion beam. Finally, the semiconductor wafers are implanted with the atomic or molecular species or ionic fragments thereof in the wafer processing device.
Ion energy is used to control junction depth in semiconductor devices. The energy of the ions that make up the ion beam determines the degree of depth of the implanted ions. High energy processes such as those used to form retrograde wells in semiconductor devices require implants of up to a few million electron-volts (MeV), while shallow junctions may only demand energies below 1 thousand electron-volts (keV), and ultra-shallow junctions may require energies as low as 250 electron-volts (eV). Typically, high current implanters (generally greater than 10 milliamps (mA) ion beam current) are used for high dose implants, while medium current implanters (generally capable up to about 1 mA beam current) are used for lower dose applications.
The continuing trend to smaller and smaller semiconductor devices requires implanters with ion sources that continue to deliver higher beam currents at lower energies. The higher beam current provides the necessary dosage levels, while the lower energy levels permit shallow implants. Source/drain junctions in complementary metal-oxide-semiconductor (CMOS) devices, for example, require such high current, low energy applications.
Conventional ion sources utilize an ionizable dopant gas that is obtained either directly from a source of a compressed gas or indirectly from a vaporized solid. Typical source elements are boron (B), phosphorous (P), gallium (Ga), indium (In), antimony (Sb), and arsenic (As). Most of these source elements are commonly used in both solid and gaseous form, except boron, which is almost exclusively provided in gaseous form. In particular applications, there is a need for low energy and ultra low energy boron implants. “Space charge” effects, however, limit the current of atomic boron that can be transported at low energy to low values, thereby reducing ion implanter productivity.
A current method used to avoid the transmission losses and so called “space charge” effects brought about by low-energy ion beam transport utilizes molecules having more than one boron atom. Transporting more than one boron atom at the same time, with a single charge, essentially reduces the implant energy of each boron atom by one over the number of atoms being transported, while increasing the effective boron current by a factor of the number of atoms per molecule. This can be achieved by generating and extracting molecules of boron, or boron clusters, with almost any number of boron atoms.
In another method, the boron atom(s) can be transported as a molecule with different elements. Again, this reduces the effective implant energy for each boron atom by a factor proportional to the molecule's total mass. The added elements of the molecule. Ideally, the added elements will simply add mass to the boron-based molecule and not affect the outcome of the implant into the crystalline structure of the silicon substrate. For example, decaborane (B10H14) can be an excellent feed material for boron implants because each decaborane molecule (B10H14) when vaporized and ionized can provide a molecular ion comprised of ten boron atoms. Such a source is especially suitable for high dose/low energy implant processes used to create shallow junctions, because a molecular decaborane ion beam can implant ten times the boron dose per unit of current as can a monatomic boron ion beam. In addition, the hydrogen molecules are not thought to adversely affect the implants of the devices. Decaborane and other borohydrides, however, can be unstable at higher temperatures, such as those found in a standard ion source. As such, to use these molecules, other methods of ionization and beam generation must be implemented.
Still another method for delivering multiple boron atoms involves using molecules that have alternate materials configured to help stabilize the borane structure. One example is the use of carborane, specifically o-carborane (C2B10H12), because the molecule is more robust and stable under the conditions found in a standard ion source. Carborane also remains stable during ionization, extraction, and transport to the wafer. The implants with this molecule, therefore, also get a dose of carbon that will modify that crystalline structure of the silicon substrate. This can be undesirable for applications in which it is important that the crystalline structure remain the same after implantation.
It would be desirable, therefore, to provide a stable boron molecule that can be implanted at low or ultra low energies without carrying any impurities to the wafer during implant.